1. Field of the Invention
This invention relates to centrifugal compressors, and more particularly to high performance centrifugal compressors using bleed apparatus in the impeller inlet region to regulate and stabilize certain operating parameters. This invention also relates to gas turbine power plants using such high performance centrifugal compressors.
2. Description of the Prior Art
Centrifugal compressors are used in various systems to provide high pressure air, which is often then directed to combustion chambers such as in a gas turbine power plant. Such compressors often experience flow instabilities during operation, especially during impeller acceleration and deceleration. The instabilities are generally caused by a shortage of air in the impeller inlet region during full-speed operation or a surplus of air during part-speed operation. A shortage of air, known as a choke condition, is typically caused by a rapid acceleration of the impeller, such that the increased compressor pumping capacity exceeds the system air intake capacity. A surplus of air, known as a surge condition, is typically caused by rapid deceleration of the impeller, leading to decreased compressor pumping capacity. Previous systems have bled air to and from the impeller inlet region to limit such instabilities.
For example, U.S. Pat. No. 4,248,566 to Chapman et al., entitled Dual Function Compressor Bleed, discloses a system employing an annular compressor shroud with a segmented annular slot configured to provide air to the impeller inlet region at full operating speed and remove air from the impeller inlet region at part operating speed. The patent discloses that such a system improves the operating efficiency of compressors by expanding surge margins and improving high speed flow capacity. One shortcoming in this design, however, is that the width of the bleed slot is fixed after production of the compressor. In addition, the annular shroud cannot be adjusted in a direction parallel to the compressor axis to set the running clearance between the impeller and shroud or to adjust the location of the bleed slot relative to the impeller. Moreover, the annular shroud is fixedly mounted to the structure near the impeller trailing edge. This configuration prevents controlled conformation of that portion of the shroud, decreasing system efficiency and increasing costs due to rubbing between the impeller and shroud.
U.S. Pat. No. 6,183,195 to Tremaine, entitled Single Slot Impeller Bleed, discloses a system employing a two-piece annular shroud where each segment is supported independently in a cantilevered manner to create an uninterrupted annular slot. While that system allows adjustment of both the running clearance and the width of the uninterrupted annular slot, those adjustments cannot be done independently. By adjusting the location of the downstream segment in the Tremaine construction, both the running clearance and the width of the uninterrupted annular slot change simultaneously. To change the running clearance but not the width of the bleed slot, thus, requires two sets of adjustments. This configuration therefore increases system complexity and assembly time. Moreover, the cantilevered mounting design requires that the shroud be fixedly mounted to the structure near the impeller trailing edge.
There is a need for a system that obviates or at least mitigates one or more of these shortcomings to allow efficient operation of a gas turbine power plant using a centrifugal compressor.